High-pressure phase and chemical equilibria of beta-diketone ligands and chelates with carbon dioxide

Supercritical fluid technology is an area of intense fundamental and applied research. More recently, supercritical fluids such as supercritical carbon dioxide have been proposed as replacement solvents for the extraction of metals. Metals are first complexed or chelated with ligands such as the β-diketone class to render them soluble in the supercritical fluid. There are several phase and chemical equilibrium steps involved in the in situ extraction of metals with β-diketones in supercritical carbon dioxide. The vapor-liquid equilibrium of a wide array of β-diketones in CO ₂ is investigated; liquid compositions, liquid molar volumes and mixture critical points are reported at 40°C and 60°C and pressures ranging from roughly 10 bar to 150 bar. The effect of substituents on the β-diketone backbone is discussed. The experimental solubility (solid-fluid equilibrium) of the metal chelates, Fe(ACAC)₃, Sm(THD)₃, and Ca(THD) ₂, in supercritical CO₂ at 40°C and 60°C, with and without cosolvents, is presented. Excess ligand, water, and cosolvents/co-contaminants all increase the solubility of metal β-diketonates. In these studies, the Peng-Robinson equation of state with van der Waals mixing rules is employed to model vapor-liquid and solid-fluid equilibrium using a completely reliable solver utilizing Interval Analysis. The model satisfactorily correlates the experimental data and could be used to aid the design of an extraction process.; The tautomerization equilibrium of β-diketones was investigated in liquids, liquid mixtures, and supercritical CO₂. The tautomerization equilibrium of β-diketones is extremely sensitive to solvent polarity. There exist several discrepancies in the literature of the tautomerization of β-diketones in supercritical CO₂, likely due to experimental difficulties.; Also, the vapor-liquid equilibrium and phase behavior of CO₂ in mixtures of the ionic liquids (3-butyl-1-methyl-imidazolium hexafluorophosphate, [C₄mim][PF₆], and tetrafluoroborate, [C₄mim][BF ₄]) and organic compounds (methanol) and water were investigated at 25°C and 40°C. At moderate pressures of CO₂, a lower critical endpoint (LCEP) is encountered and a liquid phase split occurs, producing an ionic liquid rich phase and an organic or water rich phase. Several potential separation technologies are proposed from these results that appear comparable with current separation techniques.